Alon Y. Levy Anand Rajaraman Jeffrey D. Ullman Answering Queries Using Limited External Query Processors. 69-82 1999 58 J. Comput. Syst. Sci. 1 db/journals/jcss/jcss58.html#LevyRU99 https://doi.org/10.1006/jcss.1998.1599